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Concept
Disproportionation
Summary
In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. The reverse of disproportionation, such as when a compound in an intermediate oxidation state is formed from precursors of lower and higher oxidation states, is called comproportionation, also known as synproportionation.
More generally, the term can be applied to any desymmetrizing reaction where two molecules of one type react to give one each of two different types:
2A -> A' + A''
This expanded definition is not limited to redox reactions, but also includes some molecular autoionization reactions, such as the self-ionization of water.
The first disproportionation reaction to be studied in detail was:
2 Sn^2+ -> Sn^4+ + Sn
This was examined using tartrates by Johan Gadolin in 1788. In the Swedish version of his paper he called it söndring.
Mercury(I) chloride disproportionates upon UV-irradiation:
Hg2Cl2 -> HgCl2 + Hg
Phosphorous acid disproportionates upon heating to give phosphoric acid and phosphine:
4 H3PO3 -> 3 H3PO4 + PH3
Desymmetrizing reactions are sometimes referred to as disproportionation, as illustrated by the thermal degradation of bicarbonate:
2 HCO3- -> CO3^{2}- + H2CO3
The oxidation numbers remain constant in this acid-base reaction.
Another variant on disproportionation is radical disproportionation, in which two radicals form an alkene and an alkane.
{2CH3-\underset{^\bullet}CH2 -> {H2C=CH2} + H3C-CH3}
Disproportionation of sulfur intermediates by microorganisms are widely observed in sediments.
4 S^0 + 4 H2O -> 3 H2S + SO4^{2}- + 2 H+
3 S^0 + 2 FeOOH -> SO4^{2}- + 2FeS + 2 H+
4 SO3^{2}- + 2 H+ -> H2S + SO4^{2}-
Chlorine gas reacts with dilute sodium hydroxide to form sodium chloride, sodium chlorate and water. The ionic equation for this reaction is as follows:3Cl2 + 6 OH- -> 5 Cl- + ClO3- + 3 H2O
The chlorine reactant is in oxidation state 0.
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